docs: def get_submission_ids(self, tournament=1):
          """Get dict with username->submission_id mapping.
  
          Args:
              tournament (int): ID of the tournament (optional, defaults to 1)
  
          Returns:
              dict: username->submission_id mapping, string->string
  
          Example:
              >>> NumerAPI().get_submission_ids()
              {'1337ai': '93c46857-fed9-4594-981e-82db2b358daf',
               '1x0r': '108c7601-822c-4910-835d-241da93e2e24',
               ...
               }
          """
          query = """
              query($tournament: Int!) {
                rounds(tournament: $tournament
                       number: 0) {
                  leaderboard {
                    username
                    submissionId
                  }
              }
          }
          """
          arguments = {'tournament': tournament}
          data = self.raw_query(query, arguments)['data']['rounds'][0]
          if data is None:
              return None
          mapping = {item['username']: item['submissionId']
                     for item in data['leaderboard']}
          return mapping